1. Field of the Invention
The present invention relates to a color image signal processing method which can automatically correct gradation conversion curves and color correction coefficients for converting input image signals into output image signals in a manner to finish specified points at desirable density in a color image input/output system such as a color scanner, a laser color printer, a lay-out scanner and so on.
2. Description of the Prior Art
When color images are outputted in a color image input/output system, it is often preferable to finish extremely precisely not only the whole tone but also the color tone of specified points. To meet such demands, in the case of ordinary printing scanners, for instance, a read-out head is manually moved to a specified point on an original which is pasted on an input drum to read the density at the specified point such that the final dot percentage at the point is assumed. Then, an operator should adjust gradation conversion or color correction levels by manually turning respective dials to set the final dot percentage at a desirable level. However, such manual operation requires skill and experience on the part of the operator and also takes time. The method is further easily influenced by individual preferences of the operator and seldom achieves a uniform finish.
In recent years there have been proposed methods for automatically correcting gradation conversion curves and color correction coefficients (e.g. Japanese Patent Laid-open No. 216353/1985). Most of these proposed methods, however, tend to upset the gray balance which was kept up until the correction when the gradation conversion curves are corrected. Moreover, if the density at a specified point is determined by color correction processing alone, the coefficient for the color correction often increases more than necessary to upset the color at other points. When it is necessary to finish output images precisely, they must further be corrected by the aforementioned manual operation even after they have gone through the automatic correction processing.